An aircraft generally has a flight management system (FMS) that allows the crew to record, before departure, a flight plan made up of a series of routing points. From this flight plan, the flight management system calculates and displays, on a screen, the different flight phases of the aircraft and in particular, the planned altitude level (or levels) during the en-route phase as well as data of a nature to be used by the crew such as the fuel reserve estimate.
In some cases, the air traffic controller may ask the pilot to change the altitude to fly at a lower altitude than that initially planned by the flight plan.
Currently, in case of flight at an altitude below the cruising altitude initially planned, the flight management system takes the hypothesis of imminently returning to the initial flight level to calculate the forecast amount of fuel remaining to the destination. However, keeping the flight at an altitude below the initially planned cruising altitude creates a consumption increase, and if the aircraft is not authorized to rise during a longer time than that taken into account by the flight management system, the prediction of the fuel quantity to destination is continuously updated. It decreases as the aircraft moves forward until it reaches the regulatory limits or those specified by the crew; the crew is therefore only warned that the fuel quantity regulatory limits are reached when the aircraft reaches the climb limit point towards the initially planned cruising altitude.
In order to anticipate the impact on the amount of fuel remaining to the destination of extended flight at an altitude below that initially planned for cruising, the crew can use a copy of the active flight plan (“temporary” or “secondary” flight plan) in which they modify the cruising altitude. The flight management system then recalculates the predictions on all of the new flight plan and in particular the amount of fuel to destination, which allows the crew to verify whether the amount of fuel to destination will be below the regulatory limits or those set by the crew. If this is the case, the crew must perform iterations on the position of a point for rejoining the initial altitude in the flight management system until the prediction of the amount of fuel to destination coincides with the regulatory limits (or those set by the crew). Due to the complete recalculation of the flight plan at each iteration, this process is extremely long, tiresome, and does not allow quick decision-making in response to instructions from the air traffic controller.
In fact, the crew often makes altitude changes requested by the air traffic controller without prior and precise knowledge of the impact on the amount of fuel remaining to the destination.
The object of the present invention is to propose a simple, precise onboard system resolving the aforementioned drawbacks, in particular allowing the crew to quickly assess the impact of flying at a lower level on the amount of fuel remaining to destination.